An inverter is an inverting device that electrically converts DC to AC. An inverter used in the industry receives power supplied from a commercial power supply and varies a voltage and frequency of the power and supplies the varied power to a motor. Accordingly, the inverter may control a operation speed of the motor.
An AC input power input to the inverter is rectified through a rectifying module composed of an element such as a diode, and then is smoothened by a DC link capacitor. When a power is supplied to the inverter without limiting a current while the capacitor is discharged, a large inrush current flows into the inverter. This inrush current causes the element of the rectifying module or the capacitor to be destroyed. Thus, the inverter is typically equipped with an initial charging circuit to limit the inrush current flowing into the DC link capacitor upon powering up of the inverter.
In a low-capacity inverter of smaller than or equal to 7.5 kW, the initial charging circuit is generally composed of a resistor and a switching element connected in parallel with the resistor. A main current that flows from the inverter to a load flows in this initial charging circuit. Thus, when the switching element is opened due to a failure of the initial charging circuit, there is a risk of fire due to a heat from the resistor in the initial charging circuit. Further, in an event of a short circuit occurrence, there is a possibility of a secondary failure of the initial charging circuit due to the inrush current.
Although it is possible to directly monitor a contact state of the switching element in order to detect the failure of the initial charge circuit, there is a problem that a cost of the inverter increases and a size of the inverter increases.